The present invention pertains to the sterilization art. The present invention finds particular application in conjunction with the sterilization of medical devices and will described with particular reference thereto. It is to be appreciated, however, that the invention may also find application in the sterilization, disinfecting, and liquid immersion treatment of other devices.
Sterilization is defined as the absence of all life forms including bacterial endospores which are the living organisms most resistant to known sterilants. Disinfection, by distinction, only connotes the absence of pathogenic life forms.
A sterilizer or sterilizing apparatus must demonstrate sporicidal activity which meets the standards specified in the United States Pharmacopeoia (20th revision; U.S. Pharmacopeoic Convention Inc.; Rockville, Md.) or the standards of the Association of Official Analytical Chemists (Official Methods of Analysis; 13th edition; Washington, D.C.). It is to be appreciated that establishing the absence of all life forms in sterilization is more readily documented and controlled than the elimination of pathogenic but not all life forms in disinfection.
Some high level disinfectants, such as glutaraldehyde and stabilized hydrogen peroxide, are also sporicidal. However, the six to eight hours required for 2% glutaraldehyde to achieve sterilization renders it impractical as a sterilant. Disinfection is, of course, achieved in a considerably shorter exposure time. Commercially, substantially all high level disinfectants are utilized in liquid form. Liquid sterilization and disinfecting processes immerse the item to be sterilized in a bath or vat of the sterilizing or disinfecting liquid.
Because bacterial spores are the life form which is most resistant to sterilants, they are commonly used as reproducible, stable indicators of the effectiveness of a sterilization process. In the medical industry, a sterility assurance level (SAL) of less than or equal to one chance in one million of having a contaminated item is generally regarded as the minimum acceptable level for medical devices which are designed to be used in sterile tissues of the human body. In practice, this level of assurance is obtained by establishing the exposure time required to sterilize a given quantity of bacterial endospores known to be resistant to the sterilant. Bacillus stearothermophilus is a suitable indicator for steam or moist heat sterilization and spores of bacillus subtilus are suitable indicators for dry heat or ethylene oxide sterilization. The rate of destruction of the spores at the sterilization conditions is expressed as the time required to reduce the viable spore population by 90% or 1 log. This destruction rate is commonly referenced as the D value. From the D value, which is a rate function, the time required for a given sterility assurance level can be calculated. For example, spores of Bacillus stearothermophilus typically have a D value in saturated steam of 250.degree. F. and 15 psig of two minutes. Thus, an item with 100 spores (10.sup.2) will have the spore count reduced to one spore (10.sup.0) in four minutes, i.e. a 2 log reduction. Because sterility requires an assurance level of one in a million (10.sup.-6), an additional exposure time of 12 minutes (6.times.2 minutes) or a total exposure time of 16 minutes is required for complete sterilization.
Pathogenic microorganisms, which are mostly vegetative forms of bacteria, do not have the stability to enable a D value or the equivalent to be derived after storage. There is no accompanying biological indicator applicable to pathogenic organisms which functions as a reliable, reproducible, and stable indicator of the effectiveness of the disinfection process. Accordingly, assuring that disinfection has occurred is more difficult and unreliable than assuring sterilization.
This inability to assure the effectiveness of disinfectants with biological indicators is compounded by the lack of easily measured physical parameters which demonstrate in minimal necessary conditions for disinfection. For liquid disinfectants, the active agent must be measured in individual discrete samples by a wet chemical method. Such methods are not easily automated nor do they provide continuous real time monitoring. Accordingly, liquid disinfectants are generally considered unsuitable for use by unskilled personnel.
Another problem compounding use of liquid disinfectants is that the active agent is commonly toxic to human tissue and must be removed by rinsing with water. Frequently, tap water is used as the rinse. However, the same microorganisms that are killed by the disinfectant are found in tap water and can be redeposited therefrom. Thus, the tap water rinse may defeat the disinfection process. Because sterile rinse water or saline are relatively expensive, there is a tendency for medical facilities to use a minimal amount of sterile rinse which frequently leaves a disinfectant chemical residue.
Yet another drawback to liquid disinfection processes is that the disinfected state cannot be maintained. No effective packaging or containment systems are available which can guarantee the preservation of the disinfected state of an item until use. Thus, with disinfection, a patient is apt to be exposed to both chemical residue and biological risks.
Despite the short comings of liquid disinfectants, vast quantities are used for disinfecting medical devices. Many of these medical devices are made of plastic or have complex lens systems, e.g. rigid or flexible endoscopes. The plastic elements and lens system may be destroyed or have their useful lives severely curtailed by thermal sterilization systems such as steam. Conventional ethylene oxide sterilization (which is thermally less severe than steam) requires a relatively long exposure time, on the order of three and a half hours. Ethylene oxide, which is relatively expensive compared to liquid sterilants, requires an even longer aeration time, on the order of 8-12 hours. The pressure excursions of the ethylene oxide sterilization equipment may damage lens instruments. Because ethylene oxide is a toxic, volatile gas, operator safety is a serious concern.
Accordingly, liquid sterilants rather than gaseous sterilants are commonly used for disinfection of heat sensitive and expensive medical devices in medical facilities. Liquid sterilants are rapid when used to achieve disinfection, cost effective, and do minimial damage to medical devices. However, the prior art liquid sterilant/disinfection methods and apparatus are lacking in assurance and reproducibility of disinfection, removal of chemical residues, safety, cost, and the ability to preserve the disinfected or sterile state until reuse. In addition, normal methods for the use of liquid sterilants can produce only a disinfected state because methods to produce and preserve the sterile state have not been available.
In accordance with the present invention, a new and improved liquid sterilization method and apparatus are provided which overcomes the above referenced problems and others.